In magnetic data storage systems, it is required to measure the flying height of a slider assembly near contact on a rapidly rotating rigid disk in order to verify the performance of the slider assembly. The flying height, as used herein, is the distance between the magnetic head pole and the surface of the rotating rigid disk; see, e.g., M. F. Garnier, et. al., U.S. Pat. No. 3,855,625 issued Dec. 17, 1974. The flying results from the aerodynamic effects produced by the rigid disk's rotation. The flying heights are generally less than 250 nm (10.mu.-inch) depending on the design of the slider, and may be as close as a few tens of nanometers. The trend in the art is toward very low flying heights, that is, less than 25 nanometers. In addition to the flying height, it is desirable to precisely measure the gap between the disk and the air bearing surface (ABS) over a number of points on the ABS, in order to determine the angular orientation of the slider, i.e. in terms of pitch and roll. The speed and reliability of the measurement is of particular importance, since a single slider manufacturer typically produces 200,000-500,000 slider assemblies per month.
Prior-art apparatus and methods for measuring the flying height of a slider assembly are disclosed in B. Bhushan, Tribology and Mechanics of Magnetic Storage Devices, pp. 765-797 (New York: Springer-Verlag, 1990). Some more recent developments are described in "Proceedings of the IDEMA Sub 2-micro inch Workshop", May 12, 1993. The various methods and means may be broadly divided into two classes of apparatus, referred to herein as electrical and optical flying-height testers.
Prior-art electrical flying-height testers typically employ capacitive-type sensors, see for example, G. L. Best, "Comparison of Optical and Capacitive Measurements of Slider Dynamics," IEEE Trans. on Magnetics, Vol. MAG-23, No. 5, pp. 3443-3455 (September 1987). The capacitive sensor approach is suitable for some laboratory testing but requires that a capacitive transducer be added to the slider to be tested. For production testing, this is neither practical nor cost effective. Furthermore, all of the aforementioned prior-art techniques provide poor spatial data sampling on the ABS.
Optical flying-height testers (OFHT's) are almost invariably based on interferometry. Interferometers are capable of determining the distance to an object, the topography of the object, or like physical parameters involving physical lengths (see, for example, Chapter 1 of the book Optical Shop Testing, second edition, edited by Daniel Malacara (Wiley, New York, 1992). One of the fundamental difficulties of optical techniques is that the interface between the slider ABS and a real hard disk cannot be inspected directly. Therefore, there are essentially two different types of OFHT's, those which perform a relative measurement the back side of the slider flying on a real disk, and those that use a transparent glass surrogate in place of a real hard disk.
An example of the first kind of OFHT is provided in an article entitled "Measurement of head/disk spacing with a laser interferometer," by L.-Y. Zhu, K. F. Hallamasek, and D. B. Bogy (IEEE Tran. Magn., MAG-23, 2739, 1988). The disclosed apparatus is a heterodyne interferometer capable of measuring the physical position of a plurality of points on the back side of a slider, that is, points on the side of the slider that is not in near contact with the disk. The advantages of this apparatus are that it functions with a real magnetic hard disk, and it is capable of measuring the orientation (pitch and roll) as well as the height of the slider in flight. Calibration for zero flying height is performed by landing the slider on the disk. The principle disadvantage of this kind of system is that the slider/disk interface is not observed directly, and the flying height can only be inferred from the position of the back side of the slider. Thus it must be assumed that the slider thickness and ABS shape are constant, while in fact there may be significant distortions of the slider due to mechanical and thermal stress during flight. Another disadvantage is that the back of the slider is currently not accessible on most production slider assemblies.
The first reported direct measurement of the slider/disk interface by interferometric means was reported by W. Stone in an article entitled "A proposed method for solving some problems in lubrication" (The Commonwealth Engineer, November 1921 and December 1921). Stone was obviously not working with magnetic storage media in 1921, but the essential concepts are the same ones that underlay the majority of modern OFHT's. Stone's apparatus comprises a glass disc about 125 mm in diameter so mounted that it can be rotated in a horizontal plane. A 15 mm-square block, substantially similar in function to a slider, is pressed against the lower surface of the disc through a suitable loading mechanism. Since the disk is transparent, it is possible to view the block through the disk while it is in flight. The block is illuminated through the disk with a sodium flame, which for the intended purpose acts as a nearly monochromatic light source. The reflected beam is composed of a combination of the light beam reflecting from the surface of the disk and the light beam reflecting from the block. The combination and simultaneous detection of these two reflected beams results in an interference effect related to the flying height of the block above the disk. The spacing between the block and the disk as well as the orientation of the block is deduced by visual inspection of the interference pattern as the disk varies in speed.
Modern commercial OFHT's that measure the slider/disk interface directly are based on many of the same physical principles as the apparatus invented by Stone, with the differences being principally in the type of source, detector and data processing means. A transparent surrogate disk replaces the magnetic hard disk and the interference effects at the slider ABS provide the flying height information. The interference pattern in these systems is analogous to that produced by a thin film of transparent material on a substrate. The interference pattern may therefore be said to be the result of a thin-film effect.
One form of OFHT uses the thin film effect together with a substantially monochromatic source light, as disclosed for example by G. L. Best, D. E. Horne, A. Chiou and H. Sussner, in a paper entitled "Precise optical measurement of slider dynamics," IEEE Trans. Magn. MAG-22, (1986) 1017-1019. The reflected light is modulated by the thin-film effect between the disk and the slider ABS. This modulation is periodic with the flying height, and has a period equal to one-half the wavelength of the source illumination. By introducing appropriate detection and analysis means, it is possible to track variations in the flying height by observing the modulations in intensity of the reflected light. Over certain portions of the modulation curve, it is possible by detection of the reflected intensity to determine the gap between the ABS and the disk with reasonable accuracy. Originally, such instruments involved a purely visual interpretation of the fringes. J. M. Fleischer and C. Lin were the first to use a photo-electric sensor in a monochromatic OFHT, as is described in an article entitled "Infrared laser interferometer for measuring air-bearing separation," (IBM Journal of Research and Development, 18(6), 1974, pp.529-533). A more modern example of monochromatic OFHT is described by T. Ohkubo and J. Kishegami in an article entitled "Accurate Measurement of Gas- Lubricated Slider Bearing Separation using Laser Interferometry," Trans. ASME, Vol 110, pp148-155 (January 1988). This article describes the basis of operation for the commercially available Model FM8801 and FM2000 Fly Height Testers sold in the U.S.A. by ProQuip, Inc.
In that the measurement depends on a periodic phenomenon, a disadvantage of the monochromatic OFHT is that it is not clear which interference cycle is being measured. There is consequently an ambiguity and the flying-height measurement is restricted to a range equal to one-quarter of the wavelength. A further difficulty is that there are significant ranges of flying height over which the sensitivity of the measurement is nearly zero. This aspect of the measurement method is particularly troublesome when the gap between the slider and the disk is less than 25 nm. Finally, it may be necessary to land every slider in a production test to calibrate the system for zero flying height.
In the paper entitled "A Visible Laser Interferometer for Air Bearing Separation measurement to Submicron Accuracy," by A. Niagam, Trans. ASME, Vol. 104, pp. 60-65 (1982) there is described an OFHT based on monochromatic light which also provides additional means of determining the interferometric fringe order. The additional means comprise a Xenon lamp and a circular variable wavelength filter. The lamp and wavelength filter function together as a tunable wavelength source with a range of 400 to 700 nm. As the wavelength is shifted, the interference pattern resulting from the thin-film effect is also shifted in a way which reveals the absolute flying height and thus the fringe order for the monochromatic measurement. Once the fringe order has been determined, the measurement proceeds with the monochromatic sensor at a rate of approximately 2.5 kHz.
Several other prior-art systems avoid the ambiguity problems of monochromatic interferometry by including multiple wavelengths. For example, a common form of OFHT is based on the effect of a thin film on the spectral distribution of white light, as is taught for example in the U.S. Pat. No. 4,593,368 to D. A. Fridge, et al. The apparatus in this patent comprises a computerized selector-photometer, which analyzes the wavelength-dependent modulation of white light reflected from the slider-disk interface. This technique is incorporated in commercially available products such as the line of Automatic Digital Flying Height Testers produced by Pacific Precision Laboratories, Inc. (PPL) of Chatsworth, Calif. White light interferometry has the significant advantage that there is no ambiguity in the measurement, since the spectral modulation phenomenon is not periodic with flying height. However, white light methods based on spectrometers suffer from a number of limitations, the most severe and intractable limitation being the measurement speed. This problem is compounded by the need to compensate for the phase change on reflection for as many as 171 different wavelengths (see, for example, an article entitled "Flying height measurement systems and slider absorption", by R. Pavlat, IDEMA Insight 7(5), p.1 (1994)). Finally, white light techniques are most effective for gaps greater than one-half the wavelength of the shortest wavelength used, i.e., approximately 200 nanometers, whereas the trend is towards flying heights of less than 25 nm.
In order to overcome some of the limitations of white light interferometry mentioned above, several prior-art OFHT's based on the thin-film effect use a small number of discrete wavelengths of light to improve speed and performance. In U.S. Pat. No. 5,280,340 to C. Lacey there is described a three-wavelength method of optically analyzing small spacings that comprises a high-intensity source of multiple-wavelength radiation and a detector assembly for rapid spectral analysis. The detector assembly includes wavelength discriminating beamsplitters, a filter for each individual wavelength to be measured and a high speed photodetector for each wavelength. The disclosed apparatus also comprises a mechanical assembly which is used to move the head away from the detection assembly a very small distance, on the order of 0.25 .mu.m. This mechanism is required for calibration of the apparatus, which involves measurement of the intensity of the three wavelengths while partially unloading the slider to determine the maximum and minimum intensity at each wavelength. Once the system is calibrated, it is capable of measuring flying heights at rates greater than 100 kHz. The apparatus disclosed in this patent is the basis of the Dynamic Flying Height Tester manufactured by Phase Metrics. Cambrian Pacific Technologies also markets a three-wavelength technique OFHT, the principle distinction being the use of three lasers instead of the mercury arc lamp used in the Phase Metrics system.
Although three-wavelength OFHT's are much faster than older white-light instruments, they still share many of the same limitations, the most serious of which is that the measurement sensitivity approaches zero as the flying height approaches zero. These limitations are related principally to the reliance on thin-film interference effects, which are difficult to measure when the film thickness is small. Therefore the reliance on thin-film effects is a fundamental deficiency of all of the prior-art OFHT's cited above.
The limitations of the thin-film approach are largely avoided if the reflection from the slider ABS and the reflection from the disk surface can be separated in some way, either by polarization, physical separation of the beams, or both. The apparatus disclosed in commonly-owned U.S. Pat. No. 4,606,638 to G. Sommargren uses a transparent disk is a front surface polarizer, so that the reflection from this surface can be distinguished from the reflection from the ABS. An additional advantage of the disclosed apparatus is that the entire gap is measured by a camera having a plurality of detectors, thus making it possible to determine the shape and orientation of the slider, as well as other parameters of interest that require a plurality of measurement points. However, the manufacture of the special transparent disk with the polarization coating, as taught in the Sommargren patent, can be costly and any surface imperfections might cause problems at low flying height.
Another approach to separating the interfering beams in an optical flying-height tester is disclosed in commonly-owned U.S. Pat. No. 5,218,424 to G. Sommargren. The apparatus uses two parallel beams having orthogonal polarizations. Both beams are incident on the surface of the glass disk at Brewster's angle, so that one of the beams passes completely through the disk without reflection, and the other is partially reflected from the surfaces of the disk. The beam that passes through the disk without reflection is used to illuminate the ABS. The two beams are then recombined, resulting in an interference effect that varies sinusoidally with the flying height. Since the apparatus taught in this patent is a two-beam interferometer, it is possible to measure extremely small gaps without loss of sensitivity and precision, thus eliminating one of the principle disadvantageous of systems that depend on interference effects resulting directly from multiple reflections within the gap. The disclosed apparatus also comprises an array camera for imaging the entire ABS.
Despite these advantages, the method and apparatus disclosed in commonly-owned U.S. Pat. No. 5,218,424 has significant limitations that make it an impractical tool for automated inspection of the flight characteristics of sliders used in the magnetic storage industry. These limitations include the use of an expensive, complicated, high-speed phase modulator as an essential component; a very slow data acquisition and processing rate of approximately 15 Hz, which is due in part to the method of phase modulation and the need to integrate over a full rotation of the transparent disk; a very slow determination of the dynamic flight characteristics of the slider, which is due in part to the use of a full-frame imaging camera for all measurements; a deleterious sensitivity to inhomegeneaties and distortions in the transparent disk; a deleterious sensitivity to the tip and tilt of the disk, which can introduce substantial errors in the flying height measurement; and an overall drift in the interference phase due primarily to the presence of the high-speed phase modulator, resulting in an ambiguous phase offset.
There is therefore an unmet need for an apparatus and method for accurate, high-speed characterization of the flying characteristics of sliders.